A speedometer is always installed in a vehicle such as a car, a boat, or an airplane, so that moving speed can easily be detected. However, an apparatus for detecting moving speed of a man during movement of the person has not been made available. The reason is probably because it is difficult to realize a cheap, small and portable speed detecting apparatus. On the other hand, there is a need for detecting speed while a man is walking or making sports, and it has been desired to realize an apparatus satisfying this need.
Although a speed detecting apparatus carried by a moving person has not been distributed widely, there have been proposed various types of apparatus for that purpose. For instance, the U.S. Pat. No. 4,757,714 for "Speed Sensor and Head-Mounted Data display", is disclosed a speed detecting apparatus making use of the Doppler effect of electromagnetic waves, although the availability is limited to moving sportsmen.
This example of conventional technology is a Doppler speed detecting apparatus carried by a sportsman comprising a single antenna for transmitting or receiving electromagnetic waves, a Doppler transceiver section, a section for amplifying or processing detected Doppler frequency signals, and a section for displaying speed information. Also the apparatus has a construction in which antenna directivity is at a specified angle against the ground when the Doppler speed detecting apparatus is carried by a person, in order to detect relative speed between a moving sportsman and the ground.
FIG. 11 is a view illustrating a principle when speed detection is executed by this conventional type of Doppler speed detecting apparatus. In this figure, assuming that moving speed of the sportsman is v0, and angle between a direction of the antenna directivity and the ground surface is .theta., a Doppler frequency fd obtained is expressed by the following expression; EQU fd={(2v0.multidot.f0)/C}.multidot.cos.theta. (1)
herein f0 is frequency of electromagnetic wave transmitted from a Doppler transceiver section, while C is light velocity.
As shown in expression (1), a Doppler frequency obtained by the Doppler transceiver section changes due to the angle .theta. between a direction of the antenna directivity and the ground surface. As a result, in the speed information displayed on the Doppler speed detecting apparatus, an error is generated due to the posture of a carrier and mounting angle of the apparatus, which dissatisfies users of the apparatus.
In addition, surface reflection waves of transmitted electromagnetic waves do not always provide a stable signal having a constant amplitude because the antenna directivity angle is within a certain range in a Doppler transceiver section and also because the material of the ground surface (such as lawn, earth, and asphalt) is not always constant. Namely, the Doppler transceiver section contains a noise element and receives surface reflection waves with a Doppler frequency always changing and varying in signal strength. As a result, an error sometimes is generated in speed measurement by the Doppler speed detecting apparatus.
In the "Doppler speed meter" disclosed in Japanese Patent Laid Open Publication No. 65678/1976, 2 units of horn antenna to remove a speed element in a direction perpendicular to the traveling direction are provided, but as the same frequency is used, technology such as antenna polarization to evade interference between the antennas is required. Furthermore, in a speed detecting apparatus using one type of fixed frequency, the amplitude of the surface reflection wave is not always constant because of the material and state of the ground surface, and sometimes surface reflection waves not having ample amplitude may be generated.
As described above, in the case of the conventional type of speed detecting apparatus, if a small portable speed detecting apparatus is realized by making use of the Doppler effect based on microwave or millimeter waves, an error arises in speed detection due to such causes as mounting angle of a portable speed detecting apparatus, the posture of a carrier, and material conditions of ground surface (asphalt, lawn, and earth). For this reason it is not possible to accurately report the moving speed of a carrier, making it impossible to be satisfactory to a user of the speed detecting apparatus.
Also, in a speed detecting apparatus for vehicles having two units of horn antenna provided therein to remove speed elements in a direction perpendicular to the traveling direction, as the same frequency is used, a specific circuit such as a circuit for antenna polarization for evading interference between antennas is required, and sometimes accurate speed detection may become impossible due to effects by the material and state of the ground surface.